This invention relates to a magnetron anode and a method of manufacturing the magnetron anode, and more particularly to a magnetron anode in which a pole piece is coupled in airtightness to both open ends of an anode cylinder and a method of assuring said structure of the anode.
With a magnetron unit for a microwave oven, a plurality of radially extending anode vanes are generally fitted to the inner peripheral wall of a copper anode cylinder. Resonance cavities are formed between the vanes. Both open ends of the anode cylinder are respectively fitted with a ring-shaped pole piece to conduct a magnetic flux to an interaction space defined in the anode cylinder. Recently, a magnetron anode structure is accepted in which an iron pole piece is connected in airtightness to both open ends of the anode cylinder, and the pole pieces and anode cylinder jointly constitute part of a vacuum envelope. The airtight coupling of the pole pieces to the anode cylinder has hitherto been effected by brazing or arc welding. The process of fixing pole pieces to an anode cylinder in airtightness is already disclosed in the Japanese utility model disclosure (KOKAI) No. 53-87044, Japanese utility model disclosure (KOKAI) No. 51-121164 and Japanese patent disclosure (KOKAI) No. 52-135659. With the welding process proposed in the above-listed disclosed publications, a welding annular groove is formed near both end openings of a copper anode cylinder along the outer peripheral wall thereof. Consequently a thin flange is formed at the both open ends of the copper anode cylinder. A pair of pole pieces are mounted on the flanges. The pole pieces and flanges are welded throughout the periphery of the anode cylinder by an arc welding electrode facing the joint of the paired pole pieces and flanges. It is known that the pole pieces can be welded to the aluminum anode cylinder in the same manner as set forth in the utility model disclosure (KOKAI) No. 54-125565.
With the above-mentioned welding process, the flanges of the anode cylinder and the peripheral edge of the respective pole pieces are welded together into an alloy junction. The alloy junction has a sufficiently airtight structure to enable the pole pieces to both open ends of the anode cylinder. However, the alloy junction has been found to have a low mechanical strength. The reason for this drawback is that an alloy junction formed of copper constituting the anode cylinder and iron constituting the pole piece is brittle. A higher concentration of iron renders the alloy junction more brittle. The above-mentioned known welding process tends to cause iron to be carried into the alloy junction at a higher concentration. With a microwave oven in which the intermittent actuation of a magnetron is carried out extremely often, mechanical stresses and deformation forces resulting from intermittent actuations are concentratedly applied to the welding junction of the copper anode cylinder and iron pole pieces which have different thermal expansion coefficients. As a result, the fragile welded junction is sometimes damaged due to the occurrence of cracks, eventually leading to the failure of the welded junction to preserve airtightness. Where the anode cylinder is made thin, then the welded junction will decrease in thickness, causing the above-mentioned difficulties to appear noticeably. Further, the aforesaid annular groove locally reduces the thickness of the anode cylinder, rendering the welded junction mechanically weak. Further, welding flushes project from the peripheral edge of the pole pieces, causing a magnet or magnetic flux-concentrating board set on the welded junction fail to be fitted to the surface of the pole pieces with sufficient tightness and leading to prominent irregularities in magnetic resistance.
The process of manufacturing an anode cylinder includes the process of cutting a copper tube in a prescribed length to provide the anode cylinder, and the process of folding a copper rectangular plate having a prescribed length into a cylindrical form and brazing the joints in airtightness by an alloy of 72% silver and 28% copper (the Japanese patent disclosure (KOKAI) No. 48-90464, Japanese utility model disclosure (KOKAI) No. 49-11659, Japanese utility model disclosure (KOKAI) No. 49-67545, Japanese utility model disclosure (KOKAI) No. 51-121160 and U.S. Pat. No. 4,163,921). Where pole pieces are welded to the anode cylinder produced by the latter process, then not only the aforesaid difficulties but also the undermentioned drawbacks arise.
Where the pole pieces are arc welded to the flange of the anode cylinder, the resultant heat causes the alloy brazing material applied to the joint of the anode cylinder to flow out along the annular groove, the inner wall and the joint of the anode cylinder, giving rise to the possibility of the airtightness of the joints being broken. Further, an anode cylinder constructed by folding, for example, a copper plate into the cylindrical form and brazing the joint of the folded plate has the drawback that even where the pole pieces are welded to the anode cylinder in a manner to bypass the brazed joints, still the brazed material is locally melted away, and consequently where part of the annular groove is formed in the brazed section of the joints of the anode cylinder, then the brazing material is concentratedly collected in the groove by its surface tension, resulting in the incomplete brazing of the joint of the anode cylinder. Where the pole pieces are welded to the anode cylinder, the brazing material sometimes locally boils by welding heat with the formation of air bubbles. To eliminate the occurrence of the above-mentioned difficulties, it is necessary to increase the thickness of the anode cylinder and weld pole pieces to the anode cylinder with utmost care. From the above-mentioned standpoint, it is strongly demanded to establish a method of manufacturing a magnetron anode adapted for quantity production and characterized by high reliability and sufficient mechanical strength.